This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-169212, filed Jun. 16, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a color cathode-ray tube (CRT) apparatus, and more particularly to a color CRT apparatus capable of displaying a high-quality image, with reduction in oval deformation of a beam spot on a peripheral portion of a screen.
Self-convergence in-line type color CRT apparatuses, each having an electron gun structure with a BPF (Bi-Potential Focus) type DACandF (Dynamic Astigmatism Correction and Focus) system, have now been widely used.
The electron gun structure with the BPF type DACandF system, as shown in FIG. 11, comprises three cathodes K arranged in line; a first grid G1; a second grid G2; a third grid G3 having two segments G31 and G32; and a fourth grid G4. The grids G1 to G4 are disposed in the named order from the cathodes (K) side toward a phosphor screen. Each grid has three in-line electron beam passage holes which are formed in association with the three cathodes K.
A voltage obtained by superimposing video signals upon a voltage of about 150V is applied to the cathodes K. The first grid G1 is grounded. A voltage of about 600V is applied to the second grid G2. A DC voltage of about 6 kV is applied to the first segment G31 of the third grid G3. A dynamic voltage obtained by superimposing a parabolic AC voltage component, which increases in accordance with an increase in the degree of deflection of an electron beam, upon a DC voltage of about 6 kV, is applied to the second segment G32 of the third grid G3. A voltage of about 26 kV is applied to the fourth grid G4.
An electron beam generating unit is constituted by the cathodes K, first grid G1 and second grid G2. The electron beam generating unit generates electron beams and forms an object point for a main lens. A prefocus lens is constituted by the second grid G2 and the first segment G31 and it prefocuses the electron beams generated from the electron beam generating unit. A BPF type main lens is constituted by the second segment G32 and the fourth grid G4. The BPF type main lens accelerates the prefocused electron beams toward the phosphor screen and ultimately focuses them on the phosphor screen.
Where electron beams are deflected onto a corner portion of the phosphor screen, a potential difference between the second segment G32 and the fourth grid G4 takes a minimum value and the intensity of the main lens formed therebetween lowers to a minimum. At the same time, a maximum potential difference is provided between the first segment G31 and the second segment G32, and a quadrupole lens is formed which has a focusing function in a horizontal direction and a divergence function in a vertical direction. At this time, the intensity of the quadrupole lens takes a maximum value.
Where the electron beams are deflected onto a corner portion on the phosphor screen, a distance between the electron gun structure and the phosphor screen becomes longest and an image point is formed at a farther position. In the case of the electron gun structure with the above-described BPF type DACandF system, the formation of the image point at a farther position is compensated by decreasing the intensity of the main lens. In addition, a deflection aberration caused by a pin-cushion-shaped horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field of a deflection yoke is compensated by the formation of a quadrupole lens.
In order to enhance the image quality in the color CRT apparatus, it is necessary to improve the focusing characteristics and beam spot shape on the phosphor screen. In the conventional in-line type color CRT apparatus, as shown in FIG. 12A, a beam spot 1 formed on a central area of the phosphor screen is circular but a beam spot 1 formed on a peripheral area extending from an end of a horizontal axis (X-axis) to an end of a diagonal axis (D-axis) is deformed in an oval shape along a horizontal axis (X-axis) (xe2x80x9chorizontal deformationxe2x80x9d) due to deflection aberration and a blur 2 occurs along a vertical axis (Y-axis). The image quality is thus degraded.
In order to solve this problem, in the electron gun structure with the BPF type DACandF system, the low-voltage-side grid constituting the main lens is composed of a plurality of segments, like the third grid G3, and a quadrupole lens which has a lens intensity varying dynamically in accordance with a deflection amount of the electron beam is formed between the segments. Accordingly, the blur 2 of the beam spot 1 is eliminated, as shown in FIG. 12B.
However, in the electron gun structure with the BPF type DACandF system, too, horizontal deformation occurs in the beam spot 1 formed on the peripheral area extending from the end of the horizontal axis (X-axis) to the end of the diagonal axis (D-axis), as shown in FIG. 12B. The horizontal deformation of the beam spot 1 occurs because the three electron beams generated from the in-line type electron gun structure are deflected by a non-uniform magnetic field produced by the deflection yoke which comprises a pin-cushion-shaped horizontal deflection field and a barrel-shaped vertical deflection field.
It is possible, therefore, to prevent horizontal deformation of the beam spot 1 by substantially uniformizing the shapes of both the horizontal deflection field and vertical deflection field produced by the deflection yoke. If the shapes of the deflection fields are substantially uniformized, however, three electron beams 4B, 4G and 4R will converge in front of a phosphor screen, as shown in FIG. 13. As a result, as shown in FIG. 14, a convergence error occurs and rasters 6B, 6G and 6R described on the phosphor screen are displaced from one another. In FIG. 13, a line 8 indicates the electron gun structure, and a line 9 a deflection center position.
In order to correct the convergence error, a method is known wherein an electron lens 10 having a function of a bipolar lens, whose intensity varies in synchronism with a variation in deflection fields, as shown in FIGS. 15A and 15B, is formed in the electron gun structure. According to this method, the electron lens 10 deflects the locus of each of side beams 4B and 4R by an angle xcex8 to the tube axis (Z-axis) so that they may converge on the phosphor screen 5. In this method, however, since the locus of each of the side beams 4B and 4R is deflected in front of the main lens 11, the side beams 4B and 4R do not pass through a central portion of the main lens 11. Consequently, the side beams 4B and 4R are affected by aberration of the main lens 11. As a result, a blur 2 occurs in the beam spot 1 on the phosphor screen 5, and the quality of the displayed image is degraded.
As has been described above, in order to enhance the image quality of the color CRT apparatus, the focusing characteristics and beam spot shape on the phosphor screen need to be improved.
With the conventional electron gun structure of the BPF type DACandF system, a vertical blue of the beam spot due to deflection aberration is eliminated and the beams are focused over the entire area of the phosphor screen. However, in the case of the conventional electron gun structure of the BPF type DACandF system, horizontal deformation of the beam spot formed on a peripheral area extending from an end of the horizontal axis to an end of the diagonal axis on the phosphor screen cannot be eliminated. Consequently, the horizontal deformation of the beam spot interferes with the electron beam passage holes in the shadow mask, thus causing moire, etc. and degrading the quality of display images such as characters.
The present invention has been made in order to overcome the above problems, and the object of the invention is to provide a color cathode-ray tube capable of displaying a high-quality image, while reducing an oval deformation of a beam spot on a peripheral area of a screen.
According to the present invention, in order to achieve the above object, there is provided a color cathode-ray tube apparatus comprising:
an electron gun structure having an electron beam generating unit for generating three electron beams arranged in line, the three electron beams comprising a center beam and a pair of side beams, and a main lens for accelerating the three electron beams generated by the electron beam generating unit toward a phosphor screen and ultimately focusing the three electron beams on the phosphor screen; and
a deflection yoke for generating deflection fields for deflecting the three electron beams generated from the electron gun structure,
wherein the main lens is formed by a focus electrode and an anode electrode both arranged in a direction of traveling of electron beams,
at least one additional electrode disposed between the focus electrode and the anode electrode, and
voltage application means for applying a voltage to each of the electrodes such that a voltage higher than a voltage applied to the focus electrode and lower than a voltage applied to the anode electrode is applied to the additional electrode, and in synchronism with deflection of the three electron beams by the deflection yoke, a value S, defined below, varies,
S=[(applied voltage to the additional electrode)xe2x88x92(applied voltage to the focus electrode)]/[(applied voltage to the anode electrode)xe2x88x92(applied voltage to the focus electrode)], and
wherein an angle of emission of each of the side beams emanating from the main lens varies in synchronism with a variation of the value S.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.